Manufacturing Of Cement.
Manufacturing Of Cement. by any one of the two methods : (i) Dry process, and (ii) Wet process. In both these processes the three distinct operations of (a) Mixing, Ball mill (b) Burning, Rotary kiln and (c) Grinding are carried out.
1. Dry process. In this process lime stone and clay are ground separately to tine powders and are then mixed together in the desired proportions. Water is then added to it so as to get a thick paste of which cakes are then made, dried and burnt in kilns. To the clinker obtained after burning, is added three to four per cent of gypsum and ground to very fine powder. This powder is cement ready for use.
This process is slow and costly. Also it is difficult to have the correct proportion of constituents and to do so is a cumbersome operation. The quality of cement is not so good as that of the one manufactured by the Wet process. this method has, therefore, become obsolete and the Wet process of manufacturing cement, described in details below, is widely used.
2. Wet process.
(i) Mixing. The crushed raw materials in desired proportions are fed into ball mills (Fig. 5.1). A little water to is added to it. Ball mill is a rotating steel cylinder in which there are hardened steel balls. When the mill rotates, the steel balls pulverise the raw materials which forms into a solution with water. This liquid mixture is known as slurry. This slurry is then passed into storage tanks known as silos where their proportioning is finally adjusted to ensure the correct chemical composition. Composition of raw mix can be controlled better by the wet process than in dry process. Corrected slurry is then fed into the rotary kiln for burning.
(ii) Burning. Corrected slurry is fed at the higher end of the inclined rotary kiln (Fig. 5.2) whereas from the lower end of the kiln flame is produced by injecting pulverized coal with a blast of air. Rotary kiln is a steel tube lined inside with free bricks. It is 90 to 120 metres long and from 2.5 to 3.5 meters in diameter. The kiln is mounted on rollers at a gradient of 1 in 25 to 1 in 30 and rotating once in every minute.
Slurry on entering the furnace losses moisture and forms into small lumps or “nodules”. The nodules gradually roll down passing through zones of rising temperature until they reach burning zone where they are finally burnt at 1500 to 1650ºC. At this temperature “nodules” change to clinkers. Clinkers are air-cooled in another inclined tube similar to the kiln but of lesser length.
(iii) Grinding. Grinding of the clinker is done in large tube mills which are kept cool by spraying water on them from outside. While grinding the clinker three to four per cent gypsum (Calcium sulphate) is added so as to control the setting time of cement. Finely ground cement is stored in silos from where it is drawn for packing.
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